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Highway Guardrail Post Spacing Calculator

Post count for a W-beam highway guardrail run, at the AASHTO tangent spacing or the tighter spacing a transition calls for.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
330 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Guardrail posts needed

52 posts

High confidence

Running these inputs gives 52 as the guardrail posts needed. Currently reading for United States — pick a different market above and the figures re-cast accordingly.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • Post count = ceil(guardrail length ÷ post spacing) + 1, the standard layout method for W-beam guardrail posts, commonly spaced at 6.25 ft (1.905 m) per AASHTO Roadside Design Guide standard post spacing

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Frequently asked questions

Why is a post added at the end (the +1)?
Dividing run length by spacing counts the spans between posts. A barrier terminates on a post at each end — the last span still needs something to bolt its downstream end to — which puts the count one higher than the span total.
Where does the standard 1.905 m (6.25 ft) spacing come from?
W-beam barrier develops its crash performance from rail tension and controlled post yielding together, and the tested configurations in the AASHTO Roadside Design Guide place posts at 6 ft 3 in on tangent runs. Wider spacing lets the rail deflect further than the tested envelope; the system stops behaving like the one that was crash-tested.
Does this calculator account for reduced spacing near transitions or bridge approaches?
No — this calculates post count for a uniform spacing entered by the user. Transition zones, bridge approaches, and curved runs often use tighter (e.g. half) post spacing per the project's specific design plans, which should be entered directly as the post spacing value if applicable.